1. Field of the Invention
The present invention relates to piezoelectric motors. More particularly, the present invention relates to a unique configuration for mounting piezoelectric elements in a piezoelectric wave motor.
2. Description of Related Art
Piezoelectric motors are utilized in a wide variety of applications in modern society, such as autofocusing camera lenses and automatic control units for window blinds. Piezoelectric motors are particularly well-suited for any application which requires a motor having a compact size (e.g. as small as the size of a fingertip), quiet operation, high torques at low speeds, quick response, and which is not affected by magnetic fields.
A typical design of an existing piezoelectric motor is shown in FIG. 1 designated by reference numeral 10. Motor 10 is a rotary motor that includes a disk-shaped stator 12 having a comb-tooth top surface 14 and flat bottom surface 16. Motor 10 also includes a thin piezoelectric ring 18, which is bonded to bottom surface 16 of stator 12 with an adhesive material, such as an epoxy resin. Dispersed around ring 18 are individual segments of piezoelectric ceramic which have been electrically poled in alternating opposite directions (indicated by xe2x80x9c+xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d) along an axis of poling which is perpendicular to the plane of stator 12.
Piezoelectric motor 10 further includes a disk-shaped rotor 20, here shown as a geared rotor, which, together with stator 12 and piezoelectric ring 18 bonded to the stator, are mounted onto a shaft 22 which extends upwardly from the center of a rigid base 24. A spring washer 26, bearing 28, and e-clip 30 function to hold rotor 20 in pressure contact with top surface 14 of stator 12. A thin friction liner 32 is placed between stator 12 and rotor 20 to reduce sliding energy losses during the operation of motor 10. (It is known in the art to attach friction liners to rotors and further references to rotors herein will be understood to include a possible friction liner.)
A high frequency a.c. voltage drive source 34 is also provided to drive motor 10. A first electrical lead 36 supplies a first a.c. voltage signal (typically, Vo sin xcfx89t) to a first set of poled segments on ring 18, and a second electrical lead 38 supplies a second a.c. voltage signal (Vo cos xcfx89t) to a second set of poled segments on ring 18, which are displaced along the stator from the first set as is know in the art. A third electrical lead 40 is connected to ground.
In operation, the a.c. voltage signals from drive source 34 cause the poled segments of piezoelectric material in ring 18 to expand and contract in such a manner that a traveling wave is generated in stator 12. The comb tooth top surface 14 of stator 12 amplifies this traveling wave, and the crests of the amplified traveling wave move in an elliptical motion such that a tangential force is created at the wave crests. As the wave crests contact rotor 20, this tangential force causes movement of rotor 20 to thereby drive motor 10.
Motor 10 has all of the desirable features which are generally associated with piezoelectric motors (e.g. compact size, quiet operation, high torques at low speeds, quick response, and not affected by magnetic fields); however, there are still several shortcomings associated with the design and operation of motor 10.
First, the expansions and contractions of the individual segments (i.e. individual piezoelectric ceramicsxe2x80x94typically referred to as elements) of ring 18 create alternating tensile and compressive stresses in the elements. Because piezoelectric elements are ceramics, and are typically weak in tension these alternating tension stresses promote the growth of cracks within the elements. Over time, these cracks will decrease motor reliability and may eventually lead to the failure of motor 10.
Second, motor 10 is driven by the expansions and contractions of the poled segments of piezoelectric element ring 18, which expansions and contractions are transverse to the element""s axis of poling. (Expansions and contractions transverse to the piezoelectric element""s axis of poling are commonly referred to as being in the xe2x80x9cd31 directionxe2x80x9d). It is well known in the art that expansions and contractions parallel to the element""s axis of poling (commonly referred to as the xe2x80x9cd33 directionxe2x80x9d) are approximately twice the magnitude of those in the d31 direction for a given electrical field. Thus, motor 10 does not fully utilize the piezoelectric properties of the elements.
Third, in order to transmit forces to the stator 12, piezoelectric element ring 18 is directly bonded to stator 12 such that shear stress is placed on the bond as the segments of ring 18 expand and contract in such a manner that a traveling wave is generated in stator 12. Over time, this shear stress on the bond between ring 18 and stator 12 may lead to the failure of motor 10.
Accordingly, it is an object of this invention to provide a piezoelectric motor in which the mounting of the piezoelectric elements is configured to reduce operating stresses in the piezoelectric elements by reduction of tensile stresses in the elements, shear stresses in the element bond, or both. It is a further object of this invention to provide a piezoelectric motor in which deformation of piezoelectric elements in the d33 direction may be utilized to produce motion.
In order to address these objects, the present invention utilizes a series of slots formed in the stator transverse to the desired wave motion. Piezoelectric elements are contained within these slots. Upon imposing an appropriate electric field, the elements deform, exerting forces perpendicular to the sides of the slots (i.e., in the direction of the desired wave motion). Because the elements so mounted impart forces resulting from their deformation in a direction perpendicular to the surface of the slots, shear forces imposed upon the elements are minimized or eliminated.
In a preferred aspect of the invention, piezoelectric elements contained in slots may be compressively fitted into the slots in the stator. Because the elements have an initial resting compression, the magnitude of tension experienced by these elements in operation is minimized or eliminated. Further, the elements may be compressed to such an amount that no tensile forces exist in the elements during operation.
In another preferred aspect, the piezoelectric elements contained in slots may be operated in the d33 mode. Operation in this mode maximizes the deformation achieved from an element for a given electric field, thus maximizing relative motion of the stator.
In a further preferred aspect, the individual piezoelectric elements contained in slots are stacked together to form what will be referred to as piezoelectric stacks. As the term is used herein, a piezoelectric stack consists of a plurality of piezoelectric elements mechanically connected side-to-side, with the elements being poled and electrically connected so that when an electric field is imposed, the deformation of the stacked elements is additive. Stacking may readily be accomplished where the elements are operated in the d33 mode so that contiguous portions of adjacent elements may be electrically connected (operated at the same potential).
In one aspect, the present invention may be a rotary piezoelectric motor having a design similar to the motor illustrated in FIG. 1, with the exception that the stator and piezoelectric element ring are replaced with a new stator and utilizing piezoelectric stack assemblies. The replacement stator and piezoelectric stack assemblies of the present invention generally comprise a stator, typically disk-shaped with a generally circular surface, the stator having a plurality of levers dispersed along and extending outwardly from the periphery of the stator surface. (As used herein, lever are projections either affixed or bonded to the stator or consist of part of the stator cut or formed into the stator material.) These levers define a plurality of slots between the levers, the slots extending along radials from the axis of the stator (i.e., transverse to the desired motion). Mounted within each of these slots is a piezoelectric element or stack.
In this latter embodiment, as mentioned, the levers may be constructed or manufactured independently of the stator and subsequently secured thereto, whether by mechanical, chemical, or other means. It will be appreciated that a variety of suitable lever designs may be used. For example, the levers may take the form of smooth or threaded rods operable to be securely pressure-fitted or threaded into corresponding slots in the stator. The levers, once so installed, may be removable or not. This feature provides design, cost, manufacturing, and repair advantages in that the levers may be constructed of materials and manufactured using methods different from the stator and subsequently secured thereto while being independently removable for replacement or repair. Thus, for example, the levers could be constructed of a lighter and stronger material than the stator, wherein the lever material requires a relatively high-temperature manufacturing process, and should one of the levers fail it alone may be quickly replaced rather than laboriously and inefficiently replacing the entire stator/lever unit.
In operation, the stacked piezoelectric elements expand and contract within the slots and act upon the levers in such a manner that a wave is generated in the stator. As with existing piezoelectric wave motors, the crests of the wave move in an elliptical motion such that a tangential force is created at the wave crests. As wave crests contact the rotor, the tangential force causes movement of the rotor to thereby drive the piezoelectric motor. Wave motion in a typical piezoelectric wave motor is a traveling wave, however the present invention also includes standing wave motors producing an elliptical motion at the crest of the waves.
The piezoelectric motor of the present invention is described below with regard to several specific embodiments of the stator and piezoelectric stack assembly, together referred to as a stator-piezoelectric stack assembly. It should be understood, however, that these embodiments are provided to show the variety of stator or piezoelectric stack configurations that can be utilized in a particular application, and are not intended to limit the scope of the present invention.
The present invention will be better understood from the following description of the invention, read in connection with the drawings described below.